Ensure timezone-aware timestamps in formula endpoint result

Author: cwasicki

src/frequenz/client/reporting/_client.py

@@ -164,7 +164,7 @@ class AggregatedMetric:
     def sample(self) -> MetricSample:
         """Return the aggregated metric sample."""
         return MetricSample(
-            timestamp=self._data_pb.sample.sampled_at.ToDatetime(),
+            timestamp=self._data_pb.sample.sampled_at.ToDatetime().replace(tzinfo=timezone.utc),
             microgrid_id=self._data_pb.aggregation_config.microgrid_id,
             component_id=self._data_pb.aggregation_config.aggregation_formula,
             metric=self._data_pb.aggregation_config.metric,